1. Field of the Invention
The present invention relates to a method for making a high-purity naphthalenedicarboxylic acid at low costs and a high efficiency. The resulting high-purity naphthalenedicarboxylic acid is useful as a raw material for high-performance polyester resins, i.e. polyethylene naphthalate (hereinafter referred to as PEN) and polybutylene naphthalate (hereinafter referred to as PBN). Furthermore, a dibasic acid mixture of terephthalic acid and 0.5 to 10 mass percent naphthalenedicarboxylic acid is used for preparation of a copolyester of polyethylene terephthalate (hereinafter referred to as PET) and PEN, which exhibits higher gas barrier and higher ultraviolet absorption than PET homopolymer.
2. Description of the Related Art
As is well known, PEN has excellent properties, i.e. high mechanical strength, high heat resistance, and high gas barrier; hence, PEN has a variety of actual and potential applications, for example, videotapes and video films for long time recording, advanced photo systems (APSs), heat resistant containers, tire cords, films, and bottles.
In addition, recent prospective applications of PBN are electrical insulating materials and automobile parts. A copolymer of PET and 0.5 to 10 mass percent PEN exhibits gas barrier and ultraviolet absorption, which are significantly higher than those of a PET homopolymer. Such characteristics are in favor of applications to bottles and films.
PEN, PBN, and PET/PEN copolymers are very expensive because no method for making a high-purity naphthalenedicarboxylic acid as a raw material at low costs and high efficiency is established. Accordingly, such disadvantages preclude commercial production of these polymers regardless of the above-mentioned advantages.
A well-known method for making a naphthalenedicarboxylic acid is oxidation of a dialkylnaphthalene such as dimethylnaphthalene with molecular oxygen in the presence of a catalyst, for example, Co, Mn, or Br. As shown in FIG. 1, however, the crude naphthalenedicarboxylic acid prepared by this method contains many impurities: tricarboxylic acids, i.e. trimellitic acid (TMA); dicarboxylic acids, i.e. bromonaphthalenedicarboxylic acid (Br-NDA); and monocarboxylic acid, i.e. formylnaphthoic acid (FNA), methylnaphthoic acid (MNA), and naphthoic acid. These impurities significantly impair physical properties of the PEN, PBN, and PET/PEN copolymers and cause coloring of polyester products.
Such problems can be easily understood by comparing with a method for making high-purity terephthalic acid. Terephthalic acid is prepared by oxidation of p-xylene in an acetic acid solvent. Crude terephthalic acid prepared by this process contains an impurity 4-carboxybenzaldehyde (4-CBA) that corresponds to the FNA in the crude naphthalenedicarboxylic acid.
Also, 4-CBA adversely affects physical properties of PET and cannot be removed from terephthalic acid by a solid-liquid separation process because both are contained in the solid phase. Hence, 4-CBA is removed as follows: Crude terephthalic acid is dissolved into high-temperature, high-pressure water typically at 80 atm pressure and 290xc2x0 C. and is reduced in the presence of a Pd/C catalyst into p-toluic acid, which is soluble in the liquid phase.
However, this process cannot be applied to purification of the crude naphthalenedicarboxylic acid because the naphthalenedicarboxylic acid is not substantially dissolved into the above high-temperature, high-pressure water. Furthermore, no other ordinary solvents that can dissolve the naphthalenedicarboxylic acid are found. Accordingly, it is very difficult to purify the crude naphthalenedicarboxylic acid.
Several methods for purifying a crude naphthalenedicarboxylic acid have been disclosed in order to solve these problems.
For example, Japanese Unexamined Patent Application Publication No. 10-53557 discloses a method for making the corresponding high-purity naphthalenedicarboxylic acid including reaction of a crude naphthalenedicarboxylic acid with an amine in a mixed solvent containing water, acetone, and the amine to form an amine salt of the naphthalenedicarboxylic acid, crystallization and separation of the resultant, and heating of the crystallized resultant in the presence of water to remove amine by evaporation.
Japanese Unexamined Patent Application Publication No. 7-304705 discloses a method for making the corresponding crystallized high-purity naphthalenedicarboxylic acid including dissolution of a crude naphthalenedicarboxylic acid into supercritical or subcritical water containing one of the entrainers, such as oxygen-containing organic solvents, i.e. alcohols, ketones, and ethers, cooling the solution to crystallize the target compound.
U.S. Pat. No. 5,256,817 discloses a method for making a high-purity naphthalenedicarboxylic acid by hydrogenation in the presence of Pd/C catalyst in a mixed solvent of acetic acid and water (about 90:10).
Each of these methods, however, requires an expensive mixed solvent and many hours for recovering the solvent; hence, these methods are still not suitable for commercial production of the high-purity naphthalenedicarboxylic acid.
An object of the present invention is to provide a method for making a mixture of high-purity terephthalic acid and high-purity naphthalenedicarboxylic acid from a raw mixture of crude terephthalic acid and crude naphthalenedicarboxylic acid using a known purification facility for terephthalic acid at high efficiency and low production costs.
Another object of the present invention is to provide a method for making high-purity naphthalenedicarboxylic acid at high efficiency using the same facility.
A method for making a high-purity naphthalenedicarboxylic acid according to the present invention comprises:
Step [I] of dissolving a raw mixture of crude terephthalic acid and crude naphthalenedicarboxylic acid into high-temperature high-pressure water to form a dibasic acid solution wherein the crude naphthalenedicarboxylic acid content is 0.1 to 10 mass percent of the crude terephthalic acid content, and bringing the dibasic acid solution into contact with hydrogen in the presence of a catalyst; and
Step [II] of crystallizing the resultant in the dibasic acid solution by multiple stages while reducing the temperature and the pressure for each stage and obtaining acid mixtures containing enriched naphthalenedicarboxylic acid or enriched terephthalic acid by solid-liquid separation.
In this method, only the acid mixture containing enriched naphthalenedicarboxylic acid may be separated in Step [II]. The purity of the resulting naphthalenedicarboxylic can be further increased by washing the acid mixture containing enriched naphthalenedicarboxylic acid separated in Step [II] with an alcoholic solvent.
Preferably, the alcoholic solvent comprises at least one selected from the group consisting of methyl alcohol, ethyl alcohol, and isopropyl alcohol. These alcohols may be used alone or in combination.
The temperature and pressure in Step [1] depend on the purity of the crude naphthalenedicarboxylic acid, the purity of the crude terephthalic acid, the ratio thereof in the mixture, and the target content of naphthalenedicarboxylic acid in the purified mixture. Preferably, the dibasic acid solution is brought into contact with hydrogen at a temperature in the range of 250xc2x0 C. to 320xc2x0 C. and a pressure of 40 to 130 atm pressure in Step [1].
Preferably, the amount of the hydrogen supplied during the hydrogenation in Step [1] is in the range of 1 to 10 mole percent of the total of the terephthalic acid and the naphthalenedicarboxylic acid in the raw mixture. In the hydrogenation, the catalyst preferably comprises at least one Group VIII metal that is supported on a carbon support. Preferably, the Group VIII metal is selected from Pd and Ru. Preferably, the amount of the Group VIII metal is 0.03 to 5.0 mass percent of the carbon support.
In this method, the raw mixture of crude terephthalic acid and crude naphthalenedicarboxylic acid may be prepared by simultaneously oxidizing p-xylene and 2.6-dimethylnaphthalene.
In the present invention, crude naphthalenedicarboxylic acid can be purified by hydrogenation of a mixture of crude naphthalenedicarboxylic acid and crude terephthalic acid and by multistage crystallization of the resultant using a known purification facility for crude terephthalic acid. Since a polyester copolymer polymerized using naphthalenedicarboxylic acid shows improved gas barrier and ultraviolet absorbability, the purified mixture of terephthalic acid and naphthalenedicarboxylic acid prepared by this method can be used as a dibasic acid mixture for forming the polyester copolymer having high gas barrier and high ultraviolet absorbability.